Systems employing data storage elements include, for example, video cameras and image displays. Such systems employ an addressing structure that provides data to or retrieves data from the storage elements. One system of this type to which one embodiment of the present invention is particularly directed is a general purpose flat panel display, whose storage or display elements store light pattern data.
A flat panel display comprises multiple display elements distributed throughout the viewing area of a display surface. A flat panel display system is desirable because it does not necessarily require a cathode-ray tube to develop a display image. A cathode-ray tube is undesirable because of its size, fragility, and need for high voltage drive circuitry.
One type of flat panel display system employs an addressing structure that accomplishes direct multiplexing of multiple liquid crystal cells or display elements arranged in an array. Each of the liquid crystal cells is positioned between a pair of electrical conductors that selectively apply select and deselect voltage signals across the liquid crystal cell to change its optical properties and thereby change the brightness of the image it develops. The select and deselect voltage signals are applied to the liquid crystal display by use of an ionizable gas contained within an enclosed volume, the gas functioning as an electrical switch that changes between a conducting state when it is ionized and a nonconducting state when it is nonionized.
The addressability of the liquid crystal display panel, i.e., the number of horizontal lines that can be addressed in a single frame time, is determined by the sum of the data set up time, the data capture time, and the ionizable gas decay time. The decay time, defined as the time the ionizable gas takes to change from the ionized conductive state to the nonionized nonconductive state, should generally be less than half the horizontal line address period to allow for cross-talk compensation. At a minimum, the decay time should be less than one horizontal line address time. For a 60 Hz frame rate, this implies an ionizable gas decay time of approximately 8-16 microseconds.
Flat panel display systems, such as those disclosed in U.S. Pat. No. 4,895,149 to Buzak et al., and U.S. Pat. No. 5,077,553 to Buzak, both assigned to the assignee of the present application, use helium as the ionizable gas because of its well known beneficial properties. Specifically, pure helium decays in approximately 16-24 microseconds, depending on various physical parameters such as temperature and pressure. This decay time range is sufficient for applications such as NTSC television and VGA resolution computer monitors. The decay time of pure helium is, however, insufficient for applications requiring faster addressability times such as high definition television (HDTV), which addresses 1024 lines in 16 milliseconds.
In these prior art fiat panel display systems, pure helium gas is ionized to the conductive state, i.e., becomes a mixture of charged and neutral particles, to form the electrical switch by receiving energy, in the form of electrons, from a electrode operatively associated with the helium gas. Helium gas particles decay from the ionized state to the nonionized state by recombination of helium ions with electrons in the gas or by collisions of the helium ions with the wall of the enclosed volume. As the ions recombine, the helium particles form excited neutral helium particles that further decay to form excited metastable helium particles and helium particles in the ground state.
The excited metastable helium particles cannot decay radiatively through dipole transitions but instead decay only by colliding with one another to produce helium ion-electron pairs and neutral helium atoms in the ground state with the wall of the display panel, or by higher order multi-pole transitions. Production of these helium ion-electron pairs results in a secondary ionization which keep the gas conductive, thereby lengthening the decay time. The ionized gas decay time represents the time during which the ionized gas returns to a nonionized state upon the removal of an electrode.
The decay time of the helium gas is, therefore, thought to be primarily controlled by the time-dependent density of these excited metastable helium particles in the afterglow of the initial ionization process. Shortening the decay time of the ionizable gas can be accomplished by decreasing the decay time of the excited metastable helium particles while producing fewer of the helium ion-electron pairs.
One method of shortening the decay time is to increase the diffusion rate and thereby increase the collision rate of the metastable particles. Increasing the diffusion rate entails significantly increasing the operating temperature of the liquid crystal display-a technique that is not a feasible alternative.
Another method of decreasing the decay time of the metastable particles includes adding a noble gas, such as xenon or neon, to the enclosed volume. The addition of a noble gas decreases the decay time in the following way. The addition of the noble gas decreases the density of the helium particles that form in the metastable state and thereby decreases the overall decay time of the ionizable gas mixture because it contains fewer metastable particles that must decay than does a pure helium ionized gas having the same number of particles.
The method of adding a noble gas to the ionizable gas suffers from several disadvantages. First, decreasing the density of the ionizable gas in the enclosed volume may decrease the effectiveness of the ionizable gas to act as an electrical switch. Second, collisions between the metastable particles and the noble gas occur relatively slowly because the metastable particles are not collisionally matched, i.e., they do not have similar energy levels that facilitate ease of collision. Third, all noble gases have their own metastable states. Collisions between two metastable particles of the added noble gas may also result in the formation of an ion-electron pair; thus the secondary ionization in the channel continues. Fourth, the addition of a noble gas does not significantly reduce the number of collisions between helium metastable particles that form ion-electron pairs resulting in secondary ionization.
There is a need, therefore, to accelerate the decay time of the metastables without producing free charge in the form of ion-electron pairs.